There is mounting concern about the safety of the award-winning Mexican journalist and human rights activist Lydia Cacho Ribeiro. She has been harassed, threatened and detained by police over the years for her work exposing human trafficking, child pornography and prostitution.

Amnesty International yesterday demanded that the Mexican authorities step up her protection at her home and offices in Cancun. She appears to suffering from a new and sinister level of harassment.

She has discovered she is under regular surveillance, at home and at work, by an armed man. Her life has also been threatened in online messages.

Two weeks ago she received a threat posted on her blog: "Get ready to have your throat cut, your lovely head will be left outside your apartment, let's see how brave you are."

/guardian.co.uk

Cacho has been repeatedly attacked and harassed since she published her book, The Demons of Eden, which documents allegations that powerful businessmen had been involved in child prostitution and trafficking. As a result, a well-known businessman was arrested and is facing serious criminal charges.

In December 2005, Cacho was arrested on defamation charges filed by another businessman. She was eventually acquitted.

Cacho was the 2007 winner of Amnesty International USA's Ginetta Sagan award for defenders of women and children's rights. In the 2008 British press awards, she was highly commended in the international journalist of the year category.

Mexican journalists have been under attack for the past decade. It is estimated that 50 journalists have been murdered since 2000. Last month, I reported on the deaths of two journalists. Few of the culprits are ever identified or brought to justice.